Bias for electron beam apparatus



Ju y 22, 1958 w. F. WESTENDORP I 2,344,757

BIAS FOR ELECTRON BEAM APPARATUS Filed July 2, 1956 Fig.

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BIAS FOR ELECTRON BEAM APPARATUS Willem F. Westendorp, Schenectady, N. Y., assignor to General Electric Company, a corporationof New York f Application July 2, 1956, Serial No. 595,504

j 4 Claims. (21. 315-14 This invention relates to a network for biasing electron beam apparatus and, more particularly, to such a bias network for producing a biasvoltage having a Wave form substantially unaffected by changes in the operating voltage rangeof' the apparatus. a

While this invention may take a variety of forms, it is ideally suited for biasing high energy electron beam ap- 1y higher potentials applied to a plurality of accelerating electrodes arrayed along the beam direction.

TIt is" apparent that electrons accelerated from this form offequipment would have a wide range of energies including thoseassociatcd with the relatively low-potential portions of the applied voltage and those associated with peak portions. a substantially sine-wave form, and the electron energy distribution follows the same function. Such widely dis tributed electron velocities render focussing of the beam extremely diflicult and. detract from. the efliciency of the apparatus sincethe lower. energy electrons do no useful work" and may result indeleterious heating effects;

My copendingapplication, Serial No. 518,199, now Patent No. 2,821,655,. referred to above, pertains to circuits for biasing electron beam apparatus to obtain a beam of electrons which is substantially mono-energetic. A= peaked voltagehaving the proper phase, amplitude, and periodicity with respect to the accelerating voltage is applied as a bias to the control electrode so that electrons flow in the equipment only during a peak portion of the cycle of the applied accelerating voltage. This is especially advantageous in the case of X-ray equipment and in certain forms of high energy electron beam irradiation equipment. However, such a circuit employs a fixed reference for the bias voltage, a saturable core transformer. Since this reference is independent of the operating voltage, there must be provision for controls, for the adjustment of bias angle and of phase whenever the operating voltage range m is changed, as from 500,000 volts to 1,000,000 volts. It is apparent that simplification of operating adjustments would be desirable.

Therefore, it is an object of this invention to provide a circuit to produce a bias voltage for electron beam apparatus which circuit obviates bias adjustments when the operating voltage range of the apparatus is changed.

A further object of this invention is to provide a bias circuit for electron beam apparatus which produces a bias voltage having a wave form which is independent of operating voltage changes.

According to the illustrated embodiment of this invention, a network is provided for biasing resonance trans- Generally, the accelerating voltage has 2,844,757. Pitented July 22,1958

. 2 former electron beam apparatus. The bias voltage out: put is from a voltage divider circuit including a charged capacitor as a reference voltage source. Unidirectional. conducting means serve to couple the votlage divider circuit with the resonant transformer and to vary the bias voltage in synchronism with the acceleratingvolta'ge supplied to the electron gun of the apparatus. 4

Features of this invention which are believed to be. novel are pointed out with particularity in the appended claims. However, for a better understanding of the invention, together with further objects and advantagesthereof, reference should be had to the following descripacross resistor 8 is shown as V tion. taken in conjunction with the accompanying drawing, wherein: Y i

Fig. l is a schematic diagram of a network embodying this invention as employed in conjunction with resonant transformer electron beam equipment; and Fig. 2 illustrates wave forms useful in explaining the characteristics of the network of Fig. l.

. Referring particularly to Fig. 1, there is illustrated schematically at 1 a resonance transformer to supplythe accelerating voltage for the electron gun 2. The gun com prises a continuously heated filament 3 providing a source; of electrons, a control or focussing electrode 4, and accelerating electrodes 5. 'The accelerating potentials aresupplied to the accelerating electrodes from successive taps 6 on. the resonance transformer 1. The voltage supplied by the resonant transformer is cyclically varying, sinusoidal in form, so that in the absence of a biasing circuit, the electron beam would comprise electron energies: varying from those associated with the low potential por-v tions of the accelerating voltage to those associated with.

, positive peak portions of the accelerating voltage. Biasing network 7 coupled to a portion of the resonant trans former provides the bias voltage for the control electrode 4. r The bias network comprises a: series input circuit in eluding an asymmetrically conducting device or rectifier 11 and a resistor 8 coupled across a portion of resonancetransformer 1; a voltage divider circuit including resistor 8, capacitor 9 and resistor 10for establishing a reference bias. voltage; and a parallel output circuit including. resistor 10 and a shunting rectifier 12 connected between the filament 3 and control electrode 4, to bias the control electrode 4 negatively with respect to the filament 3.. A constant charge on the capacitor 9 provides a reference voltage for the biasing action.

During positive half-cycles of the transformer output.

with respect to the filament 3, rectifier 11 is conducting and after a few cycles a constant voltage will be developed on capacitor 9. This voltage on capacitor 9 causes a potential drop across resistance 10 which is applied to the control electrode as the biasing potential. Rectifier 12 in the output circuit of the network serves,

when conducting, to shunt resistance 10 during positive pacitor 9. For purposes of illustration, the zero axis 0 for the negative output voltage V, is drawn coincident with the representation of the voltage V The voltage It may be seen that V =V +V since the voltage V on the capacitor is shared by resistors 8 and 10 when neither rectifier is conducting.

During the time interval 1 both rectifiers are conducting and the output voltage V; will be very low in magnitude and will be positive. The excess of the voltage V over the capacitor voltage V will be taken up by the potential drop across rectifier 11. During the interval t the output voltage V., very nearly assumes the form of the input wave V as illustrated by the displaced position of the zero axis for the output voltage. During time t both rectifiers block current flow and the output voltage V, equals very nearly V R /(R +R where R and R represent the resistances of resistors 8 and 10 respectively, the capacitor voltage being divided between these resistances.

It is apparent that the bias reference voltage V is directly proportional to the amplitude of the input voltage V and, therefore, a change in the operating voltage range can cause only a change in the amplitude scale of the output bias voltage V The output wave form does not depend upon the operating voltage range. Therefore, no adjustment of the biasing network parameters is required for operation of the electron beam apparatus at diflerent accelerating voltage ranges.

While the present invention has been described by reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the invention. Therefore, it is the aim of the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A network for biasing electron beam apparatus having an electron gun to produce a beam of electrons and a source of cyclically varying voltage to accelerate electrons in the beam, said network comprising an asymmetrically conducting circuit coupled to said source and including a first resistor and a first rectifier in series to provide a positive voltage proportional to the accelerating voltage, a voltage divider circuit coupled to said asymmertically conducting circuit and including said first resistor, a capacitor, and a second resistor in series to provide a reference bias voltage, and an asymmetrically conducting output circuit coupled to said voltage divider circuit and to said gun and including said second resistor and a second rectifier in parallel to apply a bias voltage to said gun in synchronism with said accelerating voltage having a wave form substantially independent of the magnitude of the accelerating voltage.

2. A network for biasing electron beam apparatus including an electron gun having an electron emitting electrode and a control electrode, and a resonant transformer coil to provide a cyclically varying voltage to the apparatus for accelerating electrons from the gun, said network comprising a voltage divider circuit including a first resistor, a capacitor, and a second resistor, said second resistor being connected between the control electrode and the emitting electrode to supply a negative bias voltage to the control electrode, a charging circuit coupled across a portion of said coil and including in series said first resistor and a first asymmetrically conducting device to produce a charge on said capacitor, and a second asymmetrically conducting device shunting said second resistor and cooperating with said first device to vary said bias voltage in synchronism with the accelerating voltage provided by the resonant transformer coil, the wave form of said bias voltage being substantially independent of the operating voltage of said equipment to provide a beam of electrons having substantially uniform energy.

3. A circuit for producing a pulse output in response to each cycle of an input voltage, wherein the time of occurrence of each pulse in a cycle and the length of each pulse is independent of the magnitude of the input volttage, said circuit comprising: asymmetric means for rectifying said input voltage, means responsive to the output of said asymmetric means for producing a reference voltage in bucking relation thereto and of a magnitude proportional to said input voltage, voltage divider means for applying a portion of said reference voltage to the output of said circuit, and means responsive to said input voltage when it exceeds said reference voltage for applying a substantially short circuit across the output of said circuit.

4. A pulsing circuit comprising: first and second input terminals, first and second output terminals, a first rectifier and a capacitor joined in series between said first input terminal and said first output terminal, a second rectifier joined between said first and second output terminals and poled in the same direction as said first rectifier, a lead connected between said second input terminal and said second output terminal, and a resistor joined at one end to said lead and at the other end to the junction between said first rectifier and said capacitor.

References Cited in the file of this patent UNITED STATES PATENTS 2,116,671 Dowsett et al May 10, 1938 2,144,518 Westendorp Jan. 17, 1939 2,456,809 Bedford et al Dec. 21, 1948 2,621,305 Little et al. Dec. 9, 1952 2,730,652 Gutton et al Jan. 10, 1956 FOREIGN PATENTS 524,226 Great Britain Aug. 1, 1940 

